The invention relates "to the industrial technique of flame spraying, formerly known as metallizing. Flame spraying is a process for applying a metal, a ceramic (metal oxide), or a mixed metal/ceramic (cermet) coating in a molten state to another metal surface (also called substrate) to form a bond between the two that will permit the coated surface (or object) to undergo further finishing operations, resist corrosion, or withstand high temperatures or other wear and stress during use. The coating to be sprayed on the substrate is commonly referred to as `flame spray material.` The flame spray material, which may be in rod, wire or powder form, is fed into a device called a `flame spray gun.` As the flame spray material passes through the gun it is reduced by gas-oxygen or electric arc flame to a molten or semi-molten state and propelled in atomized form onto the surface to be coated, such as paint is sprayed." Neaher, District Judge, Eutectic Corp. v. Metco Corp., 418 F.Supp. 1186, 1188, 191 USPQ 505, 507 (E.D.N.Y. 1976).
There exists in the flame spraying art a broad class or category of flame spray materials known as "self-fluxing," generally made so by the presence of small amounts of silicon and boron. The self-fluxing flame spray materials, often referred to as "self-fluxing alloys," are commonly based on nickel-chromium and have a high expansion coefficient (approximately 8 to 9.times.10.sup.-6 /.degree.F.). A self-fluxing alloy coating, applied by a flame spray gun to a substrate surface, properly prepared and heated to 200.degree. F. or higher, tends to contract during application; but the coating-substrate bond will be sufficiently strong to retain the coating until the next and critical operation referred to as "fusing."
Fusing in the flame spraying art requires the application of heat energy from an external source to the coated substrate surface so as to cause the coating to penetrate the surface and fuse or alloy therewith. In flame spraying literature, reference may be found to "wetting," a term of art denoting the ability of melted material to penetrate a metallic surface during application by a flame spray gun. When flame spraying a self-fluxing alloy, the wetting properties are sufficient or adequate to form a "retaining bond" but not sufficient or inadequate to form a "permanent bond." A permanent bond is essential when the base article and the coated substrate surface is intended to be further processed to predetermined dimensional tolerances, as by machining, grinding, polishing or honing.
A very widely used fusing technique is referred to as "torch fusing." The torch is a conventional gas welding instrument using appropriate oxygen-acetylene mixtures and ratios. The operator or technician is cautioned to always use a "soft" flame since a high velocity flame will cause the molten coating to flow unevenly. The work piece is evenly preheated to a temperature of from 600.degree. F. to 1000.degree. F. from a distance of 4 or 5 inches, moving the flame slowly back and forth. After the work piece is preheated, the torch nozzle is brought up to within 11/2" of the work piece starting about 1" inside the end or edge of the sprayed area. If the work piece is rotating, the nozzle may be held stationary until the coating comes up to the fusing temperature. If the work piece is stationary, the flame should be gently oscillated as the melting temperature is approached. The skilled operator will avoid localized overheating which will cause the coating to run, sag or diffuse. The skilled operator, with practice, will also be able to recognize the fusion point from visible characteristics such as a "shine" when the coating fuses.
Another fusing technique is "furnace fusing" performed in a controlled atmosphere. It is essential to avoid an oxidizing atmosphere which is deleterious to the coating during heating. Furnace temperatures of 1900.degree. F. to 1950.degree. F. may be used if furnace atmosphere is high in hydrogen and carbon monoxide. However, since the work piece should come up to fusing temperature as quickly as possible, actual temperature should be higher when the coated base article is put in the furnace. Excessive time at fusing temperature or inclination or non-horizontal orientation of the substrate surface within the furnace will each contribute to run, sag or diffusion of the coating.
The invention was conceived to improve the industrial technique of flame spraying after the coating has been applied to the substrate surface. The invention is used to improve the fusion of the coating to the substrate surface. The invention may be used to eliminate the technique of torch fusing. The invention may be used to improve the technique of furnace fusing.
The invention is intended for use after a flame spray coating has been applied to an "obscure" substrate surface. An obscure substrate surface may be of such a dimensional character, or have a location relative to other dimensional features of the base article, such that even a skilled operator will not have working room or space to use his gas torch in a proper or efficient manner. An obscure substrate surface can still (and must) receive a flame spray coating. Various modifications as to nozzle lengths, directional tips, etc., permit the flame spray gun to effectively reach even the obscure substrate surface. Nevertheless, the substrate surface is obscure in the meaning or sense of being relatively unexposed, inaccessible to, hidden from, or merely "hard to get at" insofar as a conventional torch fusing technique is concerned.
With reference to a conventional furnace fusing technique, the invention permits the use of a normal atmosphere furnace. There is no requirement that the furnace atmosphere be non-oxidizing. The substrate surface intended for fusing in a furnace may also be regarded as "obscure" in the sense that orientation or inclination thereof in the furnace is not critical. A curved, non-planar, or irregular coated surface may be fused without difficulty. Still another advantage of the invention is that various forms or shapes of flame spray coated base articles may be "ganged" or grouped for putting into the furnace since time at fusing temperature is not particularly critical as to a satisfactorily fused coating condition or result.